The Baltic Sea has a salinity gradient decreasing from fully marine ( 25) in the west to below 7 in the central Baltic Proper. Habitat-forming and ecologically dominant mytilid mussels exhibit decreasing growth when salinity 11; however, the mechanisms underlying reduced calcification rates in dilute seawater are not fully understood. Both [HCO3-] and [Ca.sup.2+ ] also decrease with salinity, challenging calcifying organisms through CaCO.sub.3 undersaturation (Î©[less than or equal to]1) and unfavourable ratios of calcification substrates ([Ca.sup.2+ ] and [HCO3-]) to the inhibitor (H.sup.+ ), expressed as the extended substrate-inhibitor ratio (ESIR). This study combined in situ monitoring of three southwest Baltic mussel reefs with two laboratory experiments to assess how various environmental conditions and isolated abiotic factors (salinity, [Ca.sup.2+ ], [HCO3-] and pH) impact calcification in mytilid mussels along the Baltic salinity gradient. Laboratory experiments rearing juvenile Baltic Mytilus at a range of salinities (6, 11 and 16), HCO3- concentrations (300-2100 Âµmol kg.sup.-1) and Ca.sup.2+ concentrations (0.5-4 mmol kg.sup.-1) reveal that as individual factors, low [HCO3-], pH and salinity cannot explain low calcification rates in the Baltic Sea. Calcification rates are impeded when Î©.sub.aragonite [less than or equal to] 1 or ESIR [less than or equal to] 0.7 primarily due to [Ca.sup.2+ ] limitation which becomes relevant at a salinity of ca. 11 in the Baltic Sea. Field monitoring of carbonate chemistry and calcification rates suggest increased food availability may be able to mask the negative impacts of periodic sub-optimal carbonate chemistry, but not when seawater conditions are permanently adverse, as observed in two Baltic reefs at salinities 11. Regional climate models predict a rapid desalination of the southwest and central Baltic over the next century and potentially a reduction in [Ca.sup.2+ ] which may shift the distribution of marine calcifiers westward. It is therefore vital to understand the mechanisms by which the ionic composition of seawater impacts bivalve calcification for better predicting the future of benthic Baltic ecosystems.